When using optical fibers to transmit optical signals, it is often necessary to direct optical signals between a plurality of optical fiber circuits or optical elements and a single optical fiber circuit or optical element, such as an optical signal detector. In order to do so, it is desirable to utilize an optical signal channel selection, or switching, device, sometimes referred to as an optical multiplexer. In one particular embodiment, for example, such a device can be arranged so as to interconnect a selected one of a plurality of transmitting optical fiber circuits to a single optical fiber circuit or optical element.
In one particular application, it is desirable to conduct light from a light source and wavelength selection portion of a spectroscopic analyzer through an optical fiber to a remote sensor and back through another optical fiber to the detector portion of the analyzer. When using heavy metal flouride glass (HMFG) optical fibers, it is advantageous to use single, small diameter fibers to connect the analyzer to the sensor because of the relatively high cost of such fibers. The use of such small diameter fibers, which carry less energy than larger diameter fibers, requires the use of a very sensitive analyzer, such as a Fourier Transform Infrared (FTIR) spectrometer which is relatively costly when compared with less sensitive analyzers. Accordingly, it is economically desirable to devise a technique for sharing a single analyzer among several sensors by using an optical signal channel selection device.
Other applications in which it is desirable to switch relatively easily and reliably from one optical signal channel, such as an optical fiber circuit, or optical element, to another occurs when switching is required between one or more circuits containing samples to be analyzed and a reference circuit, the use of such sample/reference circuitry often being required in many spectrometers in order to maintain the calibration accuracy necessary for the quantitative measurements being made.
In currently available optical fiber switches, or optical signal channel selection devices, the end of one optical fiber is physically transported so as to be aligned axially and in close proximity to the end of one of a plurality of optical fibers to which it is to be coupled. In such devices, the relative movement between the fibers is usually effected by using a stepping motor to cause an optical fiber to be physically transported in a linear motion into the desired positions for interfacial and axial alignment with each of the other fibers which are positioned in line along the direction of such linear motion.
Since the fiber cores which transmit the light are relatively small, in some cases the end of the fiber core having a diameter of less than 0.15 mm, the alignment of the end faces of the fibers must be very precisely controlled if light is to be effectively coupled between them. A lateral position error of 0.01 mm in such interface positional accuracy when using core diameters of 0.1 mm, for example, can produce about a 10% reduction in coupling efficiency. Moreover, if the angle that one fiber axis makes relative to the other fiber axis to which it is being coupled is out of alignment, the coupling efficiency is also reduced. In some applications, an axial mis-alignment of as little as 1.degree. may reduce the coupling efficiency to a level below a desired efficiency level. Such reductions can have an extremely adverse effect, particularly when providing precision quantitative spectroscopy measurements.
In addition, the techniques for moving fibers into close proximity with each other, as used in existing optical signal channel selection devices, make the small fiber core end areas quite vulnerable to changes in coupling efficiency which are caused by the presence of small particles, such as dust particles or other debris, which might lodge on the end face of one or both of the fibers. Moreover, such small diameter fibers are relatively fragile and repeated physical movement thereof may tend to subject the fibers to damage over a period of time.
It is desirable, therefore, to devise an optical signal channel selection device which will reliably and relatively easily permit selected ones of a plurality of optical fiber circuits, or elements, to be optically coupled to a single fiber circuit or optical element, for example, with a high degree of positional and axial accuracies, so as to provide high coupling efficiency, as when used for quantitative spectroscopic applications. It is further desirable that such a device avoid any vulnerability to the presence of dust, or other, particles on the fiber end faces and to avoid excessive repeated movement of the fibers so that damage which may tend to result therefrom can be avoided.